ADAM (‘A Disintegrin And Metalloprotease’) proteases catalyse the release of a range of cell surface proteins, activating receptor tyrosine kinase (RTK), Notch, cytokine-, chemokine- and adhesion signalling pathways important in normal and oncogenic development. Prominent oncogenic substrates include ligands and receptors in the Notch, erbB and Eph families, cytokines (TNFα, IL6), FASL, Slit, L-selectin and cadherins', which are all shed by one of the two closely related and widely-expressed proteases ADAM10 and ADAM17 (TACE). These proteases are also frequently over-expressed in cancers, correlating with aberrant signalling and poor cancer prognosis in cancers of the colon, lung, stomach, uterus and ovary1,2, and as potent activators of key oncogenic pathways, recognized targets for multi-pathway inhibition3,4.
ADAM10 in particular acts as principal sheddase for Notch5, Eph6,7 and certain EGFR ligands8 as well as E- and N-cadherin9. It's essential role for Notch function is highlighted by the resemblance of ADAM103 and Notch KO mice4, with embryonic lethal defects in somitogenesis, neurogenesis and vasculogenesis5. Notch signalling is triggered by binding of cell-surface-bound ligands, Delta-Like(1-4) or Jagged(1, 2), to Notch receptors (Notch1-4), which initiates ADAM-mediated shedding of both ligand10 and receptor extracellular domains (ECD)11. Shedding of the notch ECD provides the signal for γ-secretases to cleave and release the Notch intracellular domain (NICD), acting as transcriptional activator for an extensive set of genes11, regulating cell proliferation, differentiation, EMT and cell survival. De-regulated Notch signalling promotes the progression of solid cancers12, particularly by driving angiogenesis13, while mutant activated Notch is a known cause in 50% of T-ALL. However, pan-specific γ-secretase inhibitors blocking NICD release14 cause severe intestinal toxicity, likely reflecting the diversity of γ-secretase targets15. Similarly, small-molecule inhibitors blocking the ADAM protease active site failed clinical development due to lack of specificity and off-target effects, reflecting the close structural homology of this site in all MMPs4,16.
ADAMs are transmembrane proteins with an N-terminal pro-domain followed by metalloprotease (M), disintegrin (D), cysteine-rich (C), transmembrane and cytoplasmic domains3. Intriguingly, their proteolytic specificity is not simply due to a typical substrate cleavage signature, but relies on non-catalytic interactions of the substrate with ADAM D+C domains that align the protease domain for effective cleavage6, 17, 18. In addition, emerging evidence suggests that adopting latent and active ECD conformations may regulate ADAM10 and 17 activities. This concept is based on the protein architecture revealed in crystal structures of the related snake venom metalloproteinases, adopting two different conformations that are stabilized by distinct disulphide connectivity of the D+C domains19,20: a closed conformation suggested to deny substrate access, and an open conformation that allows substrate access and promotes cleavage. ADAM10/17 ECDs indeed harbour a large number of disulfide bonds21, including the conserved thioredoxin CxxC motif typical for disulfide exchange reactions catalysed by protein disulfide isomerases (PDIs)22, and mild reducing or oxidising conditions and PDI treatment are known to alter ADAM17 activity23,24. Considering that reactive oxygen species (ROS) are frequently elevated in tumours due to RTK and pro-inflammatory signalling, and are known to activate ADAM10/1723,24, an effect on protease domain orientation may explain the well-documented conundrum that kinase-dependent cytosolic signalling regulates the activity of the extracellular ADAM protease domain3,26.